In semiconductor manufacturing, accurate control of fluid flow through manufacturing tools is critical for the precise fabrication of circuits on substrates. To control fluid flow in current semiconductor manufacturing systems, a mass flow meter determines the flow rate of a fluid in the system and, if the flow rate should be adjusted, a mass flow controller opens or closes a valve accordingly. Many current systems rely on pressure differential mass flow meters, though thermal mass flow meters are becoming more common. In a pressure differential mass flow meter, two pressure sensors read the pressure drop across a constriction, which acts as a pressure loss inducing element, having a known area to calculate the flow rate of a gas based on known principles of fluid dynamics. Using the calculated gas flow rate, the mass flow controller can adjust a valve to increase or decrease the flow rate.
Prior art systems that rely on pressure differentials across a constriction typically have a limited range of operation. More particularly, the range of operation is often limited at low flow rates because the pressure differential between the two sensors becomes so small as to be indiscernible compared to system noise. Thus, for example, even if a flow controller is physically capable of controlling flows at rates of 0-100 mL per second, the controller may only be able to accurately control flows having rates of 20-100 mL per second because, beneath 20 mL per second, the pressure differential from the two pressure sensors is indiscernible.
In order to extend the working range to lower flow rates, in some prior art systems, a constriction with a smaller cross-sectional area is employed to increase the sensed pressure differential. While this may allow the flow meter to detect lower flow rates, employing a more restrictive constriction reduces the maximum flow capacity of the meter for a given fluid supply pressure and is often an unsatisfactory solution.